From the Sega Master System instruction manual.
The Year: 2089. A mysterious alien space fleet invades human occupied space. Called the Alien Syndrome, they are unlike anything mankind has ever encountered. Their ships are as large as cities. They have weapons powerful enough to destroy planets. With the entire human race threatened, the Alien Syndrome must be stopped.
A battalion of Earth Command Troopers attacks the alien fleet. But they are captured and held hostage. In desperation, Earth Command asks for volunteers to go on a rescue mission.
Only two soldiers are brave enough to step forward. They are Earth Command Troopers RICKY and MARY, the meanest combat troopers in the galaxy!
Armed with only a short range combat rifle, they must infiltrate all seven alien ships, rescue the hostages and escape...alive!
The odds are against them. The aliens can appear anytime... anywhere. The hostages are depending on RICKY and MARY...and on you!
Alien Syndrome was a video game available on multiple platforms in the mid-eighties. The original version was released in the arcades, but it was later ported to most console and computer systems of the day.
The idea behind the game is to blast all the aliens in an infested base, before the timer runs out and blows the base up (a rather popular idea, used in several later games). The details change from system to system, but it is always a top down viewpoint with a variety of aliens (some looking a bit too similar to the ones from the movie Alien).
The arcade version was available in a dedicated cabinet (from Sega of America), or in a conversion kit that was licensed by Sun Electronics. The two versions featured nearly identical sideart, although the marquee and control panel graphics were a bit different. The game boards were System 16 compatible, and the game itself was controlled with a standard 8-Way joystick and a single fire button for each player.
A brief strategy guide
Based on the Sega console version, although all versions are fairly similar.
Each level will be strewn about with comrades that need rescuing, simply touch them to bring them to safety. Be on the lookout for the Laser and fireball weapon upgrades (big "L" and "F" symbols), because they make your weapon far more effective than normal.
So, begin the level, rescue all the hostages, find a weapon upgrade, and then proceed to the exit chamber. The exit chamber will be guarded by a boss monster. I will detail how to beat those monsters below.
The game has seven levels, each with a unique graphics scheme, and end boss.
The boss on this level is "Squime", just shoot him at the bottom of his red stripe to win.
The boss on this level is "Aargh", just shoot upwards at his mouth, but avoid the other aliens in the room.
The boss on this level is "Tacapy", wait for him to open his eyes, and then fire a long stream at the space between them, he will fall quickly.
The boss on this level is "Masher", run a circular pattern around him, and shoot him between the legs whenever he stops moving.
The boss on this level is "Haggah", move slightly to the left of his firing slit, and shoot constantly, he will fall speedily.
The boss on this level is "Minosaur", shoot diagonally at the green portion of his face to win.
The boss on this level is "Mr. Mimi", face towards him and shoot upwards at his mouth (if you defeated the previous boss without dying, you will receive unlimited shots against this one).
After defeating the last boss you get to watch the ending sequence, which varies by platform, but I have included the text of the Sega Master System version here.
The ending sequence
In the year 2089 Earth Command Troopers Ricky and Mary successfully completed a mission to rescue their fellow Troopers from the invading alien fleet. In the course of their mission, the Earth Command Troopers blasted the alien space ships...and their evil leaders...out of the galaxy. Ricky and Mary received the Medal of Valor for their bravery. But the battle has only begun!
In the blackest reaches of space... millions of light years from known territory...the aliens thirst for revenge!
When they return, Ricky and Mary will be there, ready to once again fight the Alien Syndrome!